Conventionally, a shaped article, such as a shaped metallic article, is manufactured by melting a metallic or intermetallic compound, then casting the molten compound to form a shaped metallic article. The shaped metallic article typically is machined to provide the final product.
However, many intermetallic compounds, like metallic aluminides, have such high melting points that melting and casting the intermetallic compound into a shaped article is difficult to impossible. Other intermetallic compounds, like those containing beryllium, are very toxic in the powdered and molten forms, and therefore melting and casting such intermetallic compounds typically is avoided.
In addition, many intermetallic compounds are extremely hard. Shaped articles manufactured from such hard intermetallic compounds are very difficult to machine into intricate shapes. Therefore, shaped articles manufactured from high melting and/or extremely hard intermetallic compounds either are not available or are very expensive.
For example, a shaped article manufactured from a metallic aluminide can withstand very high operating temperatures. Therefore, a turbine blade made of a metallic aluminide would be useful in jet engines and would have improved performance characteristics in comparison to a present turbine blade comprising a titanium-nickel superalloy. However, metallic aluminides have very high melting points. Therefore, simply melting, casting and machining a metallic aluminide into the shape of a turbine blade is not practical.
Problems also exist in the manufacture of a dense shaped article from a ceramic or related material that is difficult to fabricate and machine by conventional techniques. The borides, carbides, niobiates, tantalates, oxide superconductors, chalcogenides, nitrides and silicides are examples of such difficult to fabricate and machine materials. Other related materials that are difficult to manufacture into intricately-shaped articles by conventional techniques include a cermet, which is a ceramic compound within a metallic matrix, like TiC/Ni (titanium carbide in nickel), and a ceramic/ceramic microcomposite, like Al.sub.2 O.sub.3 in B.sub.4 C (alumina in boron carbide).
Another problem encountered in the traditional methods of manufacturing and machining an intermetallic compound, a ceramic or a related material is proper phase formation. Often multiple phases of such materials are formed after manufacture and during cooling of the material. The formation of multiple phases is not acceptable in many materials, such as electronic materials, like iron tantalates, iron lead niobiates and yttrium barium copper oxide superconductors. It therefore would be desirable to provide a shaped article having uniform phase formation.
It would be advantageous to overcome the above-identified problems and provide a method of manufacturing an intricately-shaped article that does not rely on standard casting and machining techniques. Investigators therefore sought alternative methods of manufacturing shaped articles from advanced materials, such as metallic aluminides. SHS reaction technology has been investigated as one method of manufacturing shaped articles from materials that are difficult to melt, cast or machine, or that require formation of a uniform phase.
SHS reaction technology is based on the synthesis of an intermetallic compound, a ceramic or related material directly from elements and/or components comprising the intermetallic compound ceramic or related material. In general, an SHS reaction is highly exothermic. Therefore, after initiating an SHS reaction, the heat energy released by the SHS reaction raises the temperature of the adjacent reactant materials, i.e., the elements and/or components comprising the intermetallic compound, ceramic or related material, to a sufficient level to propagate the SHS reaction and to complete the SHS reaction.
In a typical SHS reaction, the elements present in the reaction product, or other suitable starting materials, are intimately admixed in a predetermined proportion to form a powdered precursor. For example, if the final composition is nickel aluminide, elemental nickel and elemental aluminum, in finely divided form, are admixed in the proper proportion to provide the desired nickel aluminide. An SHS reaction does not require an element as a starting reactant material however. SHS reactions have been performed, for example, between an element and an oxide. Specifically, a yttrium copper barium oxide Y.sub.3 CuBaO.sub.x) superconductor has been prepared by subjecting barium peroxide (BaO.sub.2), copper (Cu) and yttrium oxide (Y.sub.2 O.sub.3) to an SHS reaction.
The powdered precursor then is pressed into a preform of predetermined final shape. The SHS reaction then is initiated at one surface or edge of the preform, for example, by heating a surface of the preform until the activation temperature of the reaction is reached. Heating of the preform usually is accomplished by contacting a surface of the preform with a flame, a resistively-heated tungsten or Nichrome wire or a laser, or by igniting a solid state chemical ignitor.
The SHS reaction releases a sufficient amount of energy to initiate the reaction of adjacent reactant material. Accordingly, a reaction front, or zone, having a temperature up to about 4000.degree. C. progresses from the heated surface or edge throughout the preform. An SHS reaction therefore is analogous to the travel of a flame front through a long fuse, but an energy front, as opposed to a flame front, is present in an SHS reaction. In an SHS reaction, a high temperature wave passes through the preform to convert the reactant material of the preform into an intermetallic compound, a ceramic or a related material that is difficult to fabricate and machine by conventional techniques.
The synthesis of a material that is difficult to fabricate and machine, like a carbide, a boride, a silicide, a chalcogenide, a nitride, a niobiate, a tantalate, an oxide superconductor or an intermetallic compound, like nickel aluminide, by means of an SHS reaction is known. For example, Merzhanov et al. U.S. Pat. No. 3,726,643 discloses a method of producing a refractory inorganic compound from a Group IV, Group V or Group VI metal and a nonmetal, like carbon or nitrogen, by an SHS reaction.
Merzhanov et al. U.S. Pat. No. 4,161,152 discloses a method of preparing titanium carbide (TiC) by subjecting elemental titanium and elemental carbon to an SHS reaction, wherein a gaseous by-product is generated during the SHS reaction and is vented through a porous casing. The method disclosed by Merzhanov et al. illustrates a major disadvantage associated with products of SHS reactions. The generation of a gaseous by-product due to contaminants in the starting materials causes the preform to expand during the SHS reaction, and thereby provide a reaction product of relatively low density that merely approximates the original shape of the preform. Therefore, in addition to an unacceptably low density, the reaction product of an SHS reaction also requires extensive machining to provide an article having the desired final shape.
Merzhanov et al. U.S. Pat. No. 4,431,448 discloses a tungsten-free alloy of titanium, boron and carbon having a porosity of less than 1%. The alloy is compressed and densified in a separate process step after an SHS reaction is completed. Patents disclosing the synthesis of refractory materials by means of an SHS reaction include Holt U.S. Pat. Nos. 4,446,242 and 4,459,363.
W. L. Frankhouser, in the publication "Advanced Processing Of Ceramic Compounds," Noyes Data Corporation (1987), at pages 55 and 56, discloses that investigators attempted to overcome the problem of low density SHS reaction products by compressing the preform during the SHS reaction with an inert gas. This technique provided reaction products having densities of 90% to 95% of theoretical maximum density by controlling the pressure applied to the preform and restricting volume growth of the preform. However, disadvantages still remained. For example, a high density product is achieved by applying pressure after the SHS reaction. The reaction product then typically required extensive machining to provide a desired final shaped article. As previously discussed, the hardness and high melting points of many SHS reaction products makes melting, casting and machining of the SHS reaction product into a final shaped article difficult.
Therefore, the majority of reaction products formed by an SHS reaction are highly porous and are not suitable for use in applications where high strength is required. The preform of the powdered precursor generally is compacted to about 60% to about 85% of theoretical density, and little or no shrinkage, or densification, occurs during the SHS reaction. In contrast, gas evolution during the SHS reaction typically reduces the density of the reaction product. Therefore, to provide a reaction product having a sufficiently high density for many practical applications, the SHS reaction product is subsequently densified, or consolidated.
The SHS reaction product is consolidated by a post-reaction processing step, such as for example hot pressing, hot isostatic pressing (HIP) or hot forging. A post-reaction processing step requires additional operations and equipment, and adds considerable cost to the manufacturing method. In the case of HIP processing, the post-reaction step also is disadvantageous because of the need to clad the reaction product and because the process step is slow and expensive. Therefore, there is only a marginal advantage to subjecting a preform to an SHS reaction to make a porous, intermediate article of approximate net shape, which subsequently must be densified by a post-reaction consolidation step versus the alternative method of comminuting an SHS reaction product into a powder, forming a preform from the reaction product powder, when consolidating the preform by a standard powder metallurgy procedures. Each alternative method includes additional process steps that are difficult to control and that are costly to perform. Investigators therefore have sought improved methods of manufacturing a shaped article having at least 80% of theoretical maximum density, wherein the shaped article is ready for use without additional process steps. Until the method of the present invention, these methods have not been realized.
It is known that the reaction product of an SHS reaction can be partially densified by controlling the temperature of the preform such that a molten phase develops at the reaction front during the SHS reaction. Moreover, if the temperature of the preform is controlled and pressure is applied during the SHS reaction, then a high density reaction product can be made by SHS reaction technology. However, the problem of manufacturing an intricately-shaped article by an SHS reaction, wherein the shaped article requires little or no subsequent densification or machining still exists.
William M. Goldberger, in Advanced Materials & Processes, Vol. 141, No. 6, June 1992, discloses a method of densifying a metallic or ceramic preform into an article of near net shape by a combination of heat and pressure. The method disclosed by Goldberger utilizes particles having a sufficient electrical conductivity and a sufficient fracture resistance to serve both as a pressure transmitting medium to apply pressure to a preform and, simultaneously, as a resistance heating medium heating and densifying the preform.
The process disclosed by Goldberger has been used to densify powdered alloys, mullire, alumina, neodymium-iron-boron magnet materials, silicon carbide and similar alloys and ceramics. In each case, the alloy or ceramics was synthesized prior to consolidation, or densification. These methods are disclosed in Oslin U.S. Pat. Nos. 4,853,178 and 4,933,140.
Accordingly, it would be desirable to provide a method of manufacturing a shaped article from a powdered precursor by simultaneously subjecting a preform of the powdered precursor to an SHS reaction and to consolidation conditions, such that the resulting shaped article is sufficiently dense for practical use and does not require post reaction processing, such as densifying, melting, casting or machining.